hardness

n.

The quality or state of being hard, literally or
figuratively.

The cohesion of the particles on the surface of a body,
determined by its capacity to scratch another, or be itself
scratched;-measured among minerals on a scale of which diamond and talc
form the extremes.

The peculiar quality exhibited by water which has mineral
salts dissolved in it. Such water forms an insoluble compound with
soap, and is hence unfit for washing purposes.